Optical fibers may be used for sensing changes in an ambient medium. Optical sensors have been used to measure or detect changes in various parameters such as temperature, pressure, sound, refractive index and the like. Optical sensors can also be used to detect the presence of an analyte within a medium. In many cases, these characteristics are detected by monitoring the transmission (or reflection) spectrum of light as it propagates along an optical waveguide disposed within the ambient. Some optical fibers function as evanescent sensors based on the detection of changes in light propagating through an optical waveguide due to the optical mode that evanescently penetrates into the surrounding ambient medium.